No publicly available disclosure has been found which describes the removal of substantially all of Na.sub.2 TA from a dilute aqueous solution of sodium hydroxide as a step in the recovery of an aqueous solution of sodium hydroxide.
We are, however aware of the disclosure of U.S. Pat. No. 2,697,723 which is concerned with the separation of isophthalic acid and terephthalic acid mixtures by converting such phthalic acid mixture to its disodium salt by dissolving the mixture in aqueous sodium hydroxide. The mixed disodium salts are separated by dissolving the mixture in water and either evaporating solvent water until there remains only sufficient water to dissolve disodium isophthalate (1.7 part by weight water per 1.0 part by weight disodium isophthalate) or saturating the water of the solution with sodium chloride. Both of such steps precipitate least soluble disodium terephthalate substantially completely by making use of precipitation by the common ion effect. Said patent discloses that disodium terephthalate is soluble at 25.degree. C. in water to the extent of 14 weight percent but is soluble to the extent of less than 0.2 weight percent at 25.degree. C. in either saturated aqueous sodium chloride or saturated aqueous disodium isophthalate solutions (37 weight percent in water at 25.degree. C.).
From such facts a skilled chemist would not be led to use 12 to 14 weight percent sodium hydroxide to precipitate disodium terephthalate from an aqueous solution thereof. Further, the skilled chemist could not determine from the low (less than 0.2 weight percent) disodium terephthalate concentration in water saturated with sodium chloride, the required sodium ion concentration in gram atoms of sodium per liter of aqueous solution to be provided by sodium hydroxide to precipitate substantially all the disodium terephthalate dissolved in water based on the common ion effect of exceeding the solubility product.
The solubility product is applicable to dilute solutions in pure water. This invention uses relatively concentrated salt solutions. In concentrated solutions the nature of the solvent has been changed. The addition of an electrolyte to a salt solution usually increases the solubility of the salt based on the interionic attraction theory. There are deviations and the addition of some electrolytes will decrease the solubility of the salt. This patent deals with the solubility of sodium terephthalate in sodium hydroxide. The skilled chemist could not predict that sodium terephthalate would be insoluble in 14% sodium hydroxide (8% sodium) from the data that sodium terephthalate is 0.2% soluble in saturated sodium chloride (36% NaCl .tbd. 14.3% sodium).
The problem related to such removal of Na.sub.2 TA from a dilute aqueous solution of sodium hydroxide comes from the manufacture of terephthalic acid one of the reactants used in the manufacture of terephthalate polyesters suitable for film, fiber or filament production.
Terephthalic acid is produced, for the most part, by the catalytic liquid phase air oxidation of p-xylene in a stirred-tank type oxidation vessel, at an elevated temperature (e.g., 180.degree. C. to 235.degree. C.) and an elevated pressure (e.g., a gauge pressure of from 15 up to 30 kg/cm.sup.2). Most of the terephthalic acid (TA) produced from such oxidation precipitates during the oxidation so that the fluid effluent from such reaction is a suspension of solid TA in acetic mother liquor. Some of the precipitated TA adheres to the inner wall of the reaction vessel even in the best design so far devised for such stirred-tank type oxidation vessel.
The fluid oxidation effluent flows from the oxidation vessel at said elevated temperature and pressure through a valved transfer line into a multi-step (generally 2 or 3 step) cooling and decompressing system which precipitates more TA. The resulting slurry is then fed to a solid-liquid separation step (e.g., centrifugation or filtration) to separate and recover TA from the acetic acid mother liquor. TA adheres to the inner walls of the transfer lines (pipe and fittings) the cooling and decompressing vessels and the solid-liquid separation apparatus as well as acetic acid mother liquor transfer lines and receiving vessels.
The acetic acid mother liquor is subjected to indirect heat exchange in either its return to the oxidation step or to distillative recovery of acetic acid from water and oxygen-containing aromatic compounds produced during the oxidation. Here again terephthalic acid adheres to inner walls of transfer lines and heat exchange surfaces.
TA so produced may require further purification. This can be done by recrystallization from water or acetic acid solution or a combination thereof with a catalytic hydrogen treatment of the water or acetic acid solution at elevated temperature and pressure (e.g., 260.degree. C. and 73 kg/cm.sup.2 gauge pressures). Here also TA can adhere to inner walls of transfer, reaction and crystallization vessels.
In commercial operations wherein the foregoing process steps are conducted continuously, it is necessary to shutdown one or more of the steps and clean the adhering TA from the inner walls of transfer lines, heat exchangers and process vessels. Such cleaning of adhering TA can conveniently be accomplished by contacting said surfaces with hot (e.g., 100.degree. C.) dilute aqueous solution of sodium hydroxide which dissolves the adhering TA as Na.sub.2 TA.
While water at convenient, feasibly obtained temperatures, in commercial chemical operations can hold more than five to six weight percent Na.sub.2 TA in solution, it is better practice for the washing of adhering TA from transferring, heating and processing apparatus to limit the dissolved Na.sub.2 TA to 5 to 6 weight percent in the resultant aqueous wash liquor solution because such concentration even at 20.degree. C. will retain the Na.sub.2 TA in solution and prevent redeposition of Na.sub.2 TA on surfaces being cleaned. Such resultant aqueous wash liquor solution having a 5 to 6 weight percent Na.sub.2 TA content rapidly and readily forms from an aqueous wash solution containing 5 to 6 weight percent sodium hydroxide. From use of such caustic solution its sodium hydroxide content need not be depleted. Rather the 5 weight percent caustic solution should be used in sufficient amount to retain unused sodium hydroxide in the wash liquor solution at a 2 to 3 weight percent concentration.
Such wash liquor from a TA manufacturing plant can be discard into an aerobic-anaerobic waste treatment system wherein micro-organisms digest discarded organic and inorganic compounds and leave a non-polluting sludge and a non-polluting aqueous stream which readily aerates and holds dissolved oxygen without consuming dissolved oxygen.